A start button element is a button which opens a menu when clicked, and changes its appearance depending on whether that menu is still open. The menu is positioned such that it shares one edge with the button (the edge opposite the edge which the panel is docked to, if it is an appbar, or which it would be docked to, if it is not an appbar but is positioned against the edge of the screen.) There can be more than one start menu button - eg, one on each monitor - in which case only one of them appears pressed.
The !sbPopup command finds a start button element, on the same monitor as the mouse cursor if possible, and opens its menu.